1. Field of the Invention
This invention pertains to heterocyclic derivatives that are useful for treating parasitic protozoa infections in mammals, in particular bovine trichomoniasis and giardiasis.
2. Related Disclosures
Parasitic protozoa infections in mammals are widespread and difficult to prevent or remedy effectively. For example, Tritrichomonas foetus is an anaerobic protozoan parasite that causes bovine trichomoniasis in cattle; it is prevalent in cattle herds throughout much of the world, and causes a substantial loss in beef production. Gardia lamblia is an example of a water-borne zoonotic protozoan parasite; it is also found worldwide, and infection leads to severe diarrhea and growth retardation in humans.
It has been observed that parasitic protozoa lack a de novo purine nucleotide synthetic pathway (Wang, Trends Biochem. Sci. 7:354-356 (1982)). For example, T. foetus relies upon possession of hypoxanthine-guanine-xanthine phosphoribosyltransferase enzyme (HGXPRTase) in order to salvage purine bases from the host. Similarly, G. lamblia relies upon guanine phosphoribosyltransferase enzyme (GPRTase) for supplying its guanine nucleotide pool.
It is apparent that inhibition of the purine salvage pathways of the parasitic protozoa would be an effective way to block the ability of the parasites to survive in the host. However, it is important that any compounds capable of such an inhibiting effect should not interfere with the host HGPRTase. For example, in humans defects in HGPRTase are known to be responsible for gouty arthritis and a number of central nervous system disorders.
To assist in the identification of compounds that selectively inhibit purine salvage pathways, both enzymes (HGXPRTase from T. foetus and GPRTase from G. lamblia) were purified to homogeneity and partially characterized (Beck, et al., Mol. Biochem. Parasitol. 60:187-194 (1993); Aldritt, et al., J. Biol. Chem. 261:8528-8533 (1986)). The genes encoding the two enzymes were cloned, sequenced and expressed in transformed Escherichia coli to produce large quantities of recombinant enzyme proteins in their native state (Chin, et al., Mol. Biochem. Parasitol. 63:221-230 (1994); Sommer, et al., Mol. Biochem. Parasitol. 78:185-193 (1996)). The purified recombinant T. foetus HGXPRTase was crystallized and the crystal structure was determined by X-ray crystallography in the laboratory of Professor Robert Fletterick of the Department of Biochemistry and Biophysics at UCSF (Somoza, et al., Biochemistry 35:7032-7040 (1996)). Computer modeling of the active site in the enzyme molecule was initiated in the laboratory of Professor Irwin Kuntz to identify chemical compounds that conform to the dimensions of (and complement the chemistry of) the pocket and thus inhibit the enzyme function. A group of heterocyclic compounds have been found that conform to the dimensions of the pocket and complement its chemistry, and inhibit the purine salvage pathways of the parasites without affecting the mammalian HGPRTase.